Vertical-cavity surface-emitting lasers (VCSELs) emit radiation in a direction perpendicular to the plane of the p-n junction or substrate rather than parallel to the plane of the p-n junction as in the case of conventional edge-emitting diode lasers. In contrast to the astigmatic beam quality of conventional edge emitting lasers, VCSELs advantageously emit a circularly symmetric Gaussian beam and thus do not require anamorphic correction. VCSELs, moreover, can readily be made into two-dimensional laser arrays as well as be fabricated in extremely small sizes. Accordingly, two-dimensional VCSEL arrays have various applications in the fields of optical interconnection, integrated optoelectronic circuits and optical computing.
To achieve a low threshold current, VCSELs typically utilize a thin active region on the order of .lambda./4n thick or less, where .lambda. is the wavelength of the emitted light and n is the index of refraction of the active region. With such a thin active region, however, VCSELs have a single pass optical gain of approximately 1% or less, thereby requiring the use of end mirrors having reflectivities greater than 99% to achieve lasing. Such a high reflectivity is normally achieved by employing epitaxially grown semiconductor distributed Bragg reflector (DBR) mirrors.
DBR mirrors comprise alternating high and low index of refraction semiconductor layers. For a reflectivity greater than 99%, between 20-30 pairs of such alternating semiconductor layers is typically needed, depending on the difference between the refractive indices of the layers. Doped with the appropriate dopants to have opposite conductivity types, the DBR mirrors form with the active region a p-i-n structure. Current injection is facilitated by making electrical contacts to each DBR mirror such that electrons and holes traverse through the mirrors to reach the active region, where they combine and generate radiation.
Unfortunately, the VCSEL's applicability is severely limited by its low optical power output. Particularly, VCSELs have not been able to achieve comparable optical power output levels as those of edge-emitting lasers. The total power efficiency of VCSELs is presently limited to less than approximately 10%, whereas edge-emitting lasers routinely exhibit power efficiencies over 50%.
The VCSEL's low power efficiency results from two contributing factors: (1) low electrical conductivity, and (2) low optical quantum efficiency. The low-electrical conductivity is caused by the small cross-sectional area of the active region, i.e., small conduction area, and the high resistance associated with electron and hole transport perpendicularly through the multilayered DBR mirrors. The optical quantum efficiency of the VCSELs, however, is related to the optical field overlap with absorptive material within the laser cavity.
To date, all demonstrated designs of VCSELs have compromised between their optical and electronic characteristics. Designs that optimize optical quantum efficiency minimize electrical conductivity, and vice versa.
In a recent effort to address the high series resistance problem, Kwon et al. in U.S. Pat. No. 5,034,958 entitled "Front-Surface Emitting Diode" describe a VCSEL comprising a laser cavity disposed between upper and lower mirrors, with an active region sandwiched between upper and lower spacers. The lower mirror includes a semiconductor DBR, whereas the upper mirror includes a dielectric DBR. An electrical contact layer comprising one or two pairs of p-type doped GaAs/AlAs semiconductor layers which form a semiconductor DBR is disposed between the upper dielectric mirror and the upper spacer for injecting current into an upper portion of the active region.
The VCSEL design of Kwon et al. further comprises a contact region defined by implanting conductivity increasing ions into the region surrounding the cavity between the active layer and upper mirror. In this structure, electrical current only travels through one or two pairs of GaAs/AlAs semiconductor layers to reach the upper spacer and then the active region, instead of the typical 20-30 pairs in conventional VCSELs. Consequently, the series resistance for this VCSEL structure is reduced.
Despite this improved design, in comparison to edge emitting lasers, the series resistance is still high, limiting its performance. Although increasing the doping concentration in these layers, for example, from the typical 10.sup.18 /cm.sup.3 to 10.sup.20 /cm.sup.3 or 10.sup.21 /cm.sup.3, would further reduce the series resistance, such doping concentrations prohibitively increase optical absorption, reduce quantum efficiency and compromise power performance.
Another problem associated with prior art VCSELs is that they tend to lase in higher-order transverse modes, whereas single transverse mode TEM.sub.00 lasing is typically preferred.
Therefore, it is an object of this invention to reduce the series resistance of VCSELs without substantially compromising their optical quantum efficiency so as to improve their power efficiency.
It is another object of this invention to suppress higher-order transverse mode lasing within VCSELs.